The widespread use of ALS inhibitors and consequent increase of resistance to this herbicide group prompts new studies on the population dynamics of ALS resistant and susceptible biotypes to check whether the resistance evolution process may be reversed. Theoretically, without herbicide selection pressure to change the R/S allele ratio, an agronomic fitness cost is necessary. This is an important issue tackled by the one of the tasks of the four-year EU project PURE. Several ALS-resistant populations of Echinochloa crus-galli have been found in Italian maize crops, mostly where continuous maize cropping system is present. The aims of the research are: 1) to follow the evolution of R/S allele ratio when the selection pressure of ALS herbicides is removed through the introduction of crop rotation and the use of other herbicide modes of action; 2) the characterization of possible fitness costs of ALS-resistant Echinochloa crus-galli. Echinochloa crus- galli is a polyploid weed species highly competitive in maize and rice crops of the Northern Italy. A continuous maize field with a population of Echinochloa crus-galli highly cross-resistant to all ALS inhibitors had been previously identified. To verify the percentage of resistant plants, 400 seedlings were collected, transplanted in pots in the greenhouse and screened for resistance to nicosulfuron (97% resulted to be resistant). Three weeks after treatment, susceptible and resistant plants were selected and R and S seed stocks with similar genetic background were produced. The two sub-populations were checked for the resistance to four ALS chemical families through a comparative dose-response experiment. The possible fitness costs associated with this broad ALS inhibitors cross-resistance pattern was studied through a growth analysis field experiment conducted comparing the two biotypes. Plants were analyzed in non-competitive and competitive situations ( i.e. the S target plant was surrounded by plants of the R biotype and vice-versa) at four plant densities. Leaf material was sampled from each target plant (S and R) and ALS gene was amplified in order to check the presence of mutation(s) responsible for ALS inhibitors resistance. Dose-response experiment confirmed that S population is adequately controlled and R population is highly cross-resistant to all ALS inhibitors tested with R.I.s ranging from 19 for nicosulfuron to >85 for imazamox. Molecular analyses showed that a target-site resistance mechanism is involved. All resistant plants included in the growth analysis experiment possessed a double nucleotide substitution GC-AA, giving an Ala-Asn change at amino acid 122. Preliminary results of the comparative growth analysis showed different growth and development of the two biotypes: R seeds germinated later and R plants development was delayed by about one week in comparison with the S ones. S plants produced almost double the number of panicles than the R ones in non- competitive situation, the effect phased out at high competition level and no differences were observed at 20 plants/m . This effect is observable considering both panicle and seed dry weight, whereas no significant differences were observed considering the dry weights of vegetative plant parts. Of course, the results need further confirmation and the e